Britain said on Friday that relations with Russia had played a part in its refusal to hold a public inquiry into the murder in London of former KGB agent Alexander Litvinenko but denied this had been the main consideration.

Last week the government rejected a coroner's request for an inquiry, which he said could have revealed whether Russia was behind the killing, fuelling criticism that it did not want details to emerge which could damage improving ties with Moscow.

"It is true that international relations have been a factor in the government's decision-making," Home Secretary Theresa May wrote in a letter to Robert Owen, the coroner overseeing an inquest into the 2006 murder of Litvinenko, a Kremlin critic.

"An inquest managed and run by an independent coroner is more readily explainable to some of our foreign partners, and the integrity of the process more readily grasped," she said, but added that these factors had not been decisive.

Litvinenko, 43, died after drinking tea poisoned with a rare radioactive isotope, polonium-210, in a plush London hotel. From his deathbed he accused Russian President Vladimir Putin of ordering his murder, a charge Moscow has repeatedly denied.

Under British law, inquests conducted by coroners are held when a person dies unexpectedly to determine the cause of death.

Pre-inquest hearings have heard that Litvinenko, who had been granted British citizenship, had worked for Britain's MI6 intelligence service, and that the government had evidence which established a "prima facie case" Russia was behind his murder.

Owen, a senior judge appointed as coroner for the case, agreed in May to keep secret government material relating to these claims, but argued that he could not carry out a proper investigation himself, saying issues of Russian involvement and Britain's failure to protect Litvinenko were crucial.

Litvinenko's family have accused Britain of trying to cover up embarrassing details to protect lucrative business deals and to help mend ties with Moscow which sank to a post-Cold War low after the murder.

In her letter, May said the decision against an inquiry had been taken at the "highest levels of government", with ministers believing an inquest would suffice to shed light on the death.

"It is the view of the government that, despite the serious concerns you express, an inquest will go a substantial way to addressing or allaying public concern about this incident," she told Owen.

Evidence which could be heard in public would establish how the polonium was brought into Britain, who was carrying it, and the events leading up to the murder, she said.

May said an inquest was only required to determine how someone died, not the broader circumstances or who was responsible, and an inquiry would be expensive at a time austerity-hit Britain is trying to slash spending.

If "public concern remains" at the end of the inquest, she said ministers could then judge whether a further inquiry which could consider evidence in secret should be established.

Lawyers for Litvinenko's widow Marina have said they would launch a judicial review against the government's decision and said they were confident senior judges would back them.

British police and prosecutors have said there was enough evidence to bring murder charges against two former KGB agents, Andrei Lugovoy, who denies any involvement, and Dmitry Kovtun, but Moscow has resisted calls to extradite them.